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11 Aug 2012

Understanding speaker enclosure types Featured




I get questions all the time regarding subwoofer designs and types.

The world of subwoofer design is a very colorful and competitive.

There is no way to super simplify design since it is all math and physics. I have compiled all of the basics needed to understand subwoofer designs in this blog. I will write a future blog describing the newer designs and configurations such as Sub Alley and No man’s Land.

“Order” is the total number of energy storage components. The components are analogous to masses and springs. A mass stores energy in its velocity Em = 0.5*mass*velocity^2. A spring stores energy in displacement Es= 0.5*k*displacement^2, where k is the spring constant. A piston acting on a volume acts like a spring.

The driver is always 2 orders, because it is a piston and has a mass. The enclosure in the case of a sealed box just acts to stiffen the spring so it is still second order. An enclosure and port (the port acts like a piston) act as two energy storage components independent of the speaker, so a vented box is fourth order. A 4th order bandpass is just a vented box minus the contribution from one side of the driver. A 6th order bandpass has 2 orders from the driver, plus 4 from two ports and two volumes.

The sealed box has a 2nd order rolloff, the ported box has a 4th order rolloff. The 4th order bandpass has a second order rolloff below tuning and another above tuning. A 6th order bandpass box has a 4th order rolloff below the low tuning and a 2nd order rolloff above the high tuning.

Understanding "Thiele/Small parameters":

These are a group of parameters outlined by A. N. Thiele, and later R.H. Small, which can completely describe the electrical and mechanical

characteristics of a mid and low frequency driver operating in its

pistonic region.  These parameters are crucial for designing a quality

subwoofer enclosure, be it for reference quality reproduction or for



Driver free air resonance, in Hz.  This is the point at which

driver impedance is maximum.


System resonance (usually for sealed box systems), in Hz


Enclosure resonance (usually for reflex systems), in Hz


-3 dB cutoff frequency, in Hz


"Equivalent volume of compliance", this is a volume of air whose

compliance is the same as a driver's acoustical compliance Cms

(q.v.), in cubic meters


Effective diameter of driver, in meters


Effective piston radiating area of driver in square meters


Maximum peak linear excursion of driver, in meters


Maximum linear volume of displacement of the driver (product of Sd

times Xmax), in cubic meters.


Driver DC resistance (voice coil, mainly), in ohms


Amplifier source resistance (includes leads, crossover, etc.), in



The driver's Q at resonance (Fs), due to mechanical losses;



The driver's Q at resonance (Fs), due to electrical losses;



The driver's Q at resonance (Fs), due to all losses; dimensionless


The system's Q at resonance (Fc), due to mechanical losses;



The system's Q at resonance (Fc), due to electrical losses;



The system's Q at resonance (Fc), due to all losses; dimensionless


The system's Q at Fb, due to leakage losses; dimensionless


The system's Q at Fb, due to absorption losses; dimensionless


The system's Q at Fb, due to port losses (turbulence, viscosity,

etc.); dimensionless


The reference efficiency of the system (eta sub 0) dimensionless,

usually expressed as a percentage


The driver's mechanical compliance (reciprocal of stiffness), in



The driver's effective mechanical mass (including air load), in kg


The driver's mechanical losses, in kg/s


Acoustical equivalent of Cms


Acoustical equivalent of Mms


Acoustical equivalent of Rms


The electrical capacitive equivalent of Mms, in farads


The electrical inductive equivalent of Cms, in henries


The electrical resistive equivalent of Rms, in ohms


Magnetic flux density in gap, in Tesla


Length of wire immersed in magnetic field, in meters


Electro-magnetic force factor, can be expressed in Tesla-meters or,

preferably, in meters/Newton


Acoustical power


Electrical power


Propagation velocity of sound at STP, approx. 342 m/s


Density of air at STP 1.18 kg/m^3 (rho)

Understanding how speaker sensitivity affects real world SPL:

When it comes to mids and highs, efficiency (sensitivity) is a fairly

good indicator of output differences at the same power level.  When it

comes to subwoofer performance, the driver's sensitivity is irrelevant

unless you are also specifying a box volume.

An efficient sub requires a larger box to achieve equivalent extension

to a less efficient sub.  In a small box, the less efficient sub will

actually be LOUDER at low frequencies at the SAME POWER as the more

efficient sub.

Linear excursion is a very good indicator of ultimate output capability

(given sufficient power to drive the speaker to that point.)  To make

sound you must move air; therefore, the more air you move, the more

sound you make. When comparing two speakers of equal surface area, the

one with greater excursion capability will play louder given sufficient


Understanding the pros and cons of different designs:

Infinite Baffle ("free-air")


*    Advantages...

-       No box necessary!

-       This means it's usually cheaper to design and implement

in your system

*    Disadvantages...

-       The responsibility for damping cone motion rests solely

on the driver's suspension.  As fatigue sets in, this becomes

a critical issue in infinite baffle set-ups.

-       Less efficient in the sub-bass region than above

mentioned enclosures.

-       Potentially more expensive drivers than good boxable

woofer - The suspension must be extremely hearty and

long-lasting to withstand high power applications.

Sealed Box


*    Advantages...

-       Small enclosure volumes

-       Shallow (12 dB/Octave) roll off on low end

-       Excellent power handling at extremely low frequencies

-       Excellent transient response/ group delay


-       Easy to build and design

-       Forgiving of design and construction errors

*    Disadvantages...

-       Not particularly efficient

-       Marginal power handling in upper bass frequencies

-       Increased distortion in upper bass over ported design

-       When using high power and small box, magnet structure

is not in an ideal cooling environment

Ported Box


*    Advantages...

-       3-4 dB more efficient overall than sealed design

-       Handles upper bass frequencies better with less


-       Magnet is in good cooling environment

-       When properly designed, a ported box will slaughter a

sealed in terms of low frequency extension

*    Disadvantages...

-       Size 

-       Woofer unloads below Fb

-       More difficult to design/ can result in boomy, nasty

sounding bass if misaligned

Bandpass Box


*    Advantages...

-       When properly designed and implemented, can provide

superior LF extension and efficiency.

-       Cone motion is controlled more and therefore mechanical

power handling is increased.

-       Cones are physically protected from contents of trunk

flying around.

-       Output is easily channeled directly into the interior

of sedans.

*    Disadvantages...

-       Difficult to build (not recommended for newbies), and

very sensitive to misalignment due to calculation or

construction errors.

-       Their characteristic filtering often masks any

distortion that occurs as a result of amplifier clipping or

overexcursion and thus will give the user no warning that the

driver is over-stressed and about to fail.

-       Need substantial mid-bass reinforcement to make up for

narrow bandwidths in efficient alignments.

-       Transient response is largely dependent upon the

alignment chosen....wider bandwidths will result in sloppier

performance, narrower bandwidths (and thus higher effiencies)

result in better transient performance.

-       They can oft times be quite large.

Understanding magnet size:

Magnet *size* is meaningless!

Every speaker will have an optimal BL ((see Section 4.1),) product,

the field strength in the air gap multiplied by the length of the voice

coil wire in the field.

If the BL product is too low, the speaker is electrically not very well

damped (which will result in a woofer with a high Qts). A bump in

frequency response and a level drop in midband efficiency may be the

result.  If the BL product is too high, the speaker is electrically

overdamped (Low Qts woofer). A very high midband efficiency, but the

driver starts to roll of early.

An high BL product can be achieved in a number of ways: increase field

strength; or increase wire length in magnetic gap.

The increase in field strength is limited; so some manufacturers use

very thin wire for the voice coil, as such they can achieve a high BL

product with a low field strength (cheap magnet). Or they use an 8 layer

voice coil... needless to say that electrical power handling will

decrease enormously.

Long stroke woofers, having only a part of the voice coil in the air

gap, need a very high field strength to achieve a high BL product. Often

this means a big magnet as well...

Use magnet size as an indication, but as nothing more than that.

Last modified on Sunday, 12 August 2012 11:28
Jim Martin

Jim Martin

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